Add support for optional help text on widgets

[pcdavid]

In Sirius Desktop's EEF-based property views, it is possible to associate a help text on widgets that can display additional text in a tooltip.
We should also support this kind of mechanism in our Forms (including in View-based forms).

Tasks

• Shape written
• ADR written
• Core support for AbstractWidget[Description].helpTextProvider and hasHelpText flag.
• Implement the GraphQL Query to invoke a widget's helpTextProvider.
• Frontend: display the "?" hint for widgets with hasHelpText (need more discussion on the icon and layout)
• Frontend: invoke the GraphQL Query and display the result (as a tooltip)
• View: add WidgetDescription.helpTextExpression
• View: create a proper helpTextProvider from the expression during the View conversion.
• Frontend: display the "?" hint in the FormDescriptionEditor version of the widgets (if the helpTextExpression is defined)
• Add tests
• Update documentation and CHANGELOG with a screenshot/animation.

[pcdavid]

#1651 mentions the use of MUI's input adornments instead of an icon at the same level as the widget's title. I'll have a look but it's not clear that it would be applicable to all kinds of widgets in a uniform way (e.g. radio buttons, or more gererally custom widgets).